1. FIELD OF THE INVENTION
This invention relates to surface acoustic wave (SAW) devices, and in particular to variable dispersive SAW devices capable of producing signals having different frequency-time slopes.
2. DESCRIPTION OF THE PRIOR ART
Signals whose frequency varies with time are commonly called chirp signals and are usually generated in one of two ways; either by applying a delay which varies with the frequency of a signal, or by applying a frequency shift which varies with time. The first technique is termed time compression (or dispersion) and the second technique is termed frequency compression (or dispersion), one being the Fourier analogue of the other. A known publication describing the basic piezoelectric element utilized as a surface acoustic wave device and interdigital transducers forming dispersive delay lines or chirp filters is entitled "Hughes-Fullerton Capability Profile and Applications of Acoustic Surface Wave Devices", published by Hughes Aircraft Company, May 1971.
In co-pending UK Patent Application No. 39376/76 there is described a SAW dispersive filter (chirp filter) comprising two SAW dispersive delay lines connected in series, both having a cubic, or a higher order phase variation characteristic with frequency, a variable frequency oscillator, and two mixers for mixing the oscillator output with signals entering and leaving one of the delay lines. In such filters the dispersive delay, and hence the frequency-time slope of the chirp signals produced, can be varied by varying the frequency of the variable frequency oscillator.